Data Sheet AD7903
TERMINOLOGY
Integral Nonlinearity Error (INL)
INL refers to the deviation of each individual code from a line
drawn from negative full scale through positive full scale. The
point used as negative full scale occurs ½ LSB before the first
code transition. Positive full scale is defined as a level 1½ LSB
beyond the last code transition. The deviation is measured from
the middle of each code to the true straight.
Differential Nonlinearity Error (DNL)
In an ideal ADC, code transitions are 1 LSB apart. DNL is the
maximum deviation from this ideal value. It is often specified in
terms of resolution for which no missing codes are guaranteed.
Offset Error
Offset error is the difference between the ideal midscale voltage
(that is, 0 V) and the actual voltage producing the midscale
output code (that is, 0 LSB).
Offset Error Match
It is the difference in offsets, expressed in millivolts between the
channels of a multichannel converter. It is computed with the
following equation:
Offset Matching = VOFFSETMAX − VOFFSETMIN
where:
VOFFSETMAX is the most positive offset error.
VOFFSETMIN is the most negative offset error.
Offset matching is usually expressed in millivolts with the full-
scale input range stated in the product data sheet.
Gain Error
The first transition (from 100 … 00 to 100 … 01) should occur
at a level ½ LSB above nominal negative full scale (−4.999981 V
for the ±5 V range). The last transition (from 011 … 10 to
011 … 11) occurs for an analog voltage that is 1½ LSB below the
nominal full scale (4.999943 V for the ±5 V range). The gain
error is the deviation of the difference between the actual level
of the last transition and the actual level of the first transition from
the difference between the ideal levels.
Gain Error Match
It is the ratio of the maximum full scale to the minimum full
scale of a multichannel ADC. It is expressed as a percentage of
full scale using the following equation:
%100
2
×
+
−
=
MIN
MAX
MIN
MAX
FSRFSR
FSRFSR
MatchingGain
where:
FSRMAX is the most positive gain error of the ADC.
FSRMIN is the most negative gain error.
Spurious-Free Dynamic Range (SFDR)
SFDR is the difference, in decibels (dB), between the rms
amplitude of the input signal and the peak spurious signal.
Effective Number of Bits (ENOB)
ENOB is a measurement of the resolution with a sine wave
input. It is related to SINAD by the following formula:
ENOB = (SINADdB − 1.76)/6.02
ENOB is expressed in bits.
Noise Free Code Resolution
Noise free code resolution is the number of bits beyond which it
is impossible to distinctly resolve individual codes. It is calculated
as follows:
Noise Free Code Resolution = log2(2N/Peak-to-Peak Noise)
Noise free code resolution is expressed in bits.
Effective Resolution
Effective resolution is calculated as follows:
Effective Resolution = log2(2N/RMS Input Noise)
Effective resolution is expressed in bits.
Total Harmonic Distortion (THD)
THD is the ratio of the rms sum of the first five harmonic
components to the rms value of a full-scale input signal and is
expressed in decibels (dB).
Dynamic Range
Dynamic range is the ratio of the rms value of the full scale to
the total rms noise measured with the inputs shorted together.
The value for dynamic range is expressed in decibels (dB). It is
measured with a signal at −60 dBFS to include all noise sources
and DNL artifacts.
Signal-to-Noise Ratio (SNR)
SNR is the ratio of the rms value of the actual input signal to the
rms sum of all other spectral components below the Nyquist
frequency, excluding harmonics and dc. The value for SNR is
expressed in decibels (dB).
Signal-to-(Noise + Distortion) (SINAD) Ratio
SINAD is the ratio of the rms value of the actual input signal to
the rms sum of all other spectral components below the Nyquist
frequency, including harmonics but excluding dc. The value for
SINAD is expressed in decibels (dB).
Aperture Delay
Aperture delay is the measure of the acquisition performance. It
is the time between the rising edge of the CNVx input and
when the input signal is held for a conversion.
Transient Response
Transient response is the time required for the ADC to accurately
acquire its input after a full-scale step function is applied.
Rev. B | Page 13 of 28